Trucks, particularly heavy duty trucks, commonly employ multiple speed counter-shaft type mechanical transmissions having up to at least 18 different torque multiplication ratios. The large number of ratios is needed to enable a fully loaded truck to perform a variety of necessary tasks, including low speed maneuvering in forward and reverse gears as required for moving about freight yards and for loading and unloading tasks, accelerating from a dead stop, accelerating while rolling, maintaining speed while on a grade, and so on. The large number of gears means that there is a frequent need for shifting. Manual gear shifting, as well as the selection of the correct gear, are tasks which require considerable operator experience to consistently execute optimally. Increasingly, transmissions for heavy-duty trucks are being automated. However, even with a large number of ratios, and automated shifting, the engine speed varies with the speed of the vehicle when the vehicle is in a given gear ratio. This means that the engine's operating parameters must be compromised to accommodate the anticipated range of engine operating speeds. An engine that could be tuned to operate at a single engine speed could be tuned to operate much more efficiently. Shifting decreases the operating efficiency of the vehicle, as there can is typically a dip in vehicle speed during the shift when the engine is momentarily disconnected from the drive wheels, and a subsequent need to bring the vehicle back to its target speed.
It is desired to provide a transmission which provides a full ratio coverage, yet minimizes the need for shifting. It is also desirable to provide the engine with a narrower anticipated speed operating range so as to permit the optimization of engine parameters. Continuously variable transmissions or CVTs provide a means of operating a vehicle engine at a continuous speed across a wide range of vehicle speeds. CVT employ a wide variety of variators to achieve the desired continuous variation in torque and speed multiplication. Know variator forms include pump-motor systems, belt or chain and pulley systems, motor-generator systems and others. For a vehicle to get the maximum efficiency benefit of operating its engine at a constant speed, it is important that the variator as efficient as possible. The pulley-chain type variator has been shown to be among the most efficient models. However, pulley chain variators available today generally have a limited torque capacity and, for a heavy duty vehicle such as a commercial truck, would be unable to sustain all of the engine power all of the time. One known approach of the limitations to the use of CVTs employing chain and pulley type variators has been to reduce the duty cycle to which such variators are exposed. This has typically been accomplished by using the variators in a power splitting mode at low or launch speeds to avoid exposing the variator to what is where it is expected to be the maximum level of torque the transmission will see. The transmission will then operate in the variator only mode at higher speeds to provide the desired vehicle speed range coverage. However, for certain applications such as line-haul trucks where the vehicle spends most of its driving time at relatively elevated speeds, operating in the variator only mode for long distances has the undesired effect of reducing fuel economy. It is desired to provide a CVT and a CVT control system which operates the CVT in a power splitting mode at high vehicle speeds.